I think if it actually 24 or 25 seconds depends on what happens when the shot clock operator presses the button. I've run the shot clock for college games, but I've never noticed if it:
1. Immediately goes to 23, because it's really counting the hundredths of seconds behind the scenes and just not showing them, or
2. It waits a second before it goes to 23
My theory is that when the shot clock goes from say 10 to 9, when it hits 9, it's really closer to 9.99 because it's counting the partial
seconds but just not showing them. So, that when it goes from showing single digits 7,6,5 and then to 4.99 it sits on 5 for a full
second because it's really counting from 5.99 to 5.00 behind the scenes.
So... I'll bet when the NBA shot clock operator hits the start button on the shot clock, it immediately shows 23, but in reality it's closer to 23.9,
and the clock isn't showing the partial seconds.